What are Core Web Vitals? A plain-English explanation

Google measures three things about every website and uses them as a ranking factor. They're called Core Web Vitals, and most business owners have never heard of them. Here's what they are, why they matter, and how to check yours in about 30 seconds.

The three metrics

LCP (Largest Contentful Paint) - how long until the biggest thing on the page appears. Usually a hero image or a large heading. Target: under 2.5 seconds.

Think of it as: how long does a visitor stare at a blank or half-loaded screen before they see something meaningful?

INP (Interaction to Next Paint) - how long from when someone taps a button to when the page actually responds. Target: under 200 milliseconds.

Think of it as: does the site feel snappy when you interact with it, or is there a noticeable lag?

CLS (Cumulative Layout Shift) - how much the page jumps around while loading. Buttons that move just as you're about to tap them. Text that shifts down when an ad loads above it. Target: under 0.1.

Think of it as: is the page stable, or does it feel like the ground is moving under your feet?

Why they matter for your business

Two reasons, one practical and one strategic:

Practical: slow sites lose customers. Studies consistently show that conversion rates drop roughly 20% for every additional second of load time. If your site takes 6 seconds to load on mobile and a competitor's takes 2, you're losing business to them -not because your service is worse, but because your website is.

Strategic: Google uses Core Web Vitals as a ranking signal. All three metrics feed into how Google decides where to place your site in search results. A fast site with good CWV doesn't automatically rank first, but a slow site with poor CWV is competing with a handicap.

How to check yours

Go to pagespeed.web.dev. Paste your website URL. Click "Analyze." Look at the mobile results.

You'll see a Performance score out of 100, plus individual scores for each Core Web Vital. Green means passing, orange means needs improvement, red means failing.

Here's what the overall scores roughly mean:

• 90-100: Excellent. Your site is fast and well-built.
• 70-89: Good but with room to improve. Usually one or two specific issues dragging the score down.
• 50-69: Mediocre. Your site is noticeably slow on mobile and it's probably costing you rankings and conversions.
• Below 50: Poor. The site needs a technical intervention, not a tweak.

What causes bad scores

Almost always one of these:

• Huge images that haven't been compressed or resized for the web
• Too much JavaScript from plugins, analytics tools, chat widgets, and ad trackers
• Slow hosting that adds hundreds of milliseconds to every request
• Unoptimised fonts loading multiple weights from external servers
• Layout shifts from images without dimensions, late-loading ads, or dynamically injected content

What to do about it

If your score is below 70 on mobile, you have two options:

Quick fixes (often gets you from 50 to 70): compress images, remove unused plugins, switch to faster hosting, lazy-load below-fold content. A developer can usually do this in a day.

Rebuild (gets you from any score to 90+): move to a modern stack (Next.js, Astro, SvelteKit) with proper image optimisation, minimal JavaScript, and edge hosting. This is what TillerLabs does -our sites consistently score 95+ on mobile.

If you want to know where your site stands and what it would take to fix it, request a free health check. We'll run a full performance audit and give you a clear breakdown.

Keep reading